Tsukuno is a minor character in the story of Rewrite. She's Akane Senri's secretary, and Shimako's caretaker. We're shown a different side of her during the Terra route, albeit under the name of Nagai (長居), where she shows how much she lacks confidence and how depressed she is, to the point where she doesn't mind laying her own life to destroy the world. (Source: Rewrite Wiki)